Immersion lithographic apparatus with a projection system having an isolated or movable part

ABSTRACT

A lithographic projection apparatus is disclosed where at least part of a space between a projection system of the apparatus and a substrate is filled with a liquid by a liquid supply system. The projection system is separated into two separate physical parts. With substantially no direct connection between the two parts of the projection system, vibrations induced in a first of the two parts by coupling of forces through the liquid filling the space when the substrate moves relative to the liquid supply system affects substantially only the first part of the projection system and not the other second part.

The present application is a continuation of U.S. patent applicationSer. No. 10/890,389, filed Jul. 14, 2004 now U.S. Pat. No. 7,483,118,now allowed, which claims priority to European patent application no. EP03254699.6, filed Jul. 28, 2003, the entire contents of each of theforegoing applications herein fully incorporated by reference.

FIELD

The present invention relates to a lithographic projection apparatus anda device manufacturing method.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning devices include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the undiffracted light can        be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning device can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat. No.        5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent        applications WO 98/38597 and WO 98/33096, which are incorporated        herein by reference. In the case of a programmable mirror array,        the support structure may be embodied as a frame or table, for        example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “projection lens”; however, this term should bebroadly interpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTpatent application WO 98/40791, incorporated herein by reference.

It has been proposed to immerse the substrate in a lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system.)

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid in a localized area between the final element of the projectionsystem and the substrate (the substrate generally has a larger surfacearea than the final element of the projection system). One way which hasbeen proposed to arrange for this is disclosed in PCT patent applicationWO 99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thefinal element relative to the substrate, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thefinal element relative to the substrate, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.There are other ways of putting the localized area solution into effect,see for example U.S. patent application Ser. No. 10/705,783.

A problem that may occur is that when a substrate is scanned normalforces and shear forces are coupled through the liquid and passed onto afinal element of the projection system. This may cause unwantedvibrations in the projection system. Furthermore, the projection systemis typically attached to a reference frame which carries sensitiveinstruments required for accurate alignment during the imaging process.Any vibrations of the projection system may therefore have an adverseeffect on the accuracy of the apparatus and on the quality of thedevices manufactured.

SUMMARY

Accordingly, it would be advantageous, for example, to reduce thetransmission of forces coupled by the liquid when the substrate movesrelative to the projection system in an immersion lithography apparatus.

According to an aspect, there is provided a lithographic apparatus,comprising:

an illumination system arranged to condition a radiation beam;

a support structure configured to hold a patterning device, thepatterning device being capable of imparting the radiation beam with apattern;

a substrate table configured to hold a substrate;

a projection system arranged to project the patterned radiation beamonto a target portion of the substrate, the projection system comprisingtwo separate physical parts that are decoupled, wherein each partcomprises an optical element of the projection system; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate, with a liquid.

By separating a projection system into two separate parts which aredecoupled, i.e. there is no direct mechanical connection between them,any forces coupled through the liquid primarily act only on a secondpart of the two parts of the projection system. The first part of thetwo parts of the projection system is isolated from the second part andsubstantially no force coupled through the liquid is exerted upon it.Therefore, any vibrations arising because of coupled forces through theliquid primarily affect only the second part of the projection system.Thus, vibration sensitive optical elements may be placed in therelatively vibration free first part of the projection system to improvethe quality of the projected image.

In an embodiment, the parts of the projection system are separated at alocation between two lens elements having a large curvature radiusand/or between two lens elements where the patterned beam is collimated.By separating the projection system at such a location, the alignmentbetween the two parts of the projection system may be less sensitive tolateral movements i.e. those movements horizontal to the substrate. Thismay make the apparatus simpler to construct because lateral tolerancesare correspondingly lower. In an embodiment, those elements having alarger curvature radius are chosen over the rest of the optical elementsin the projection system.

In an embodiment, the projection system is a telecentric lens system andthe parts are separated in a pupil plane of the lens system. Atelecentric system comprises a first and second lens group separated bya pupil (or aperture). A pupil plane (or aperture plane) (which includesany substantially conjugate plane) creates an ideal location at which toseparate a telecentric lens system into two parts. At the pupillocation, the lens system is less sensitive to movements in theillumination direction i.e. movements perpendicular to the substrate.Therefore, in such an apparatus the tolerance for alignment between thefirst and second parts of the projection system may not required to beunmanageably high and construction may be simplified.

In an embodiment, the apparatus further comprises:

a sensor configured to establish a position between a first opticalelement in the first part of the projection system and a second opticalelement in the second part of the projection system;

an actuator configured to vary the position between the first and secondoptical elements; and

a controller configured to control the actuator on the basis of outputfrom the sensor to maintain a predetermined position between the firstand second optical elements.

For design reasons, it may not always be possible to separate aprojection system between lens elements having a large curvature radius,or at the aperture of a telecentric lens system. If a projection systemis split at an arbitrary location, it is likely that it will beintolerant to variations in the separation of the elements, for example,in the beam direction i.e. the optical axis which is perpendicular tothe substrate. This construction allows a predetermined position betweentwo lens elements, one in the first part and one in the second part, tobe maintained. The apparatus can also be applied when a lens system issplit between lenses having a large curvature radius, or at the aperturelocation of a telecentric lens system to further improve the accuracy oftheir alignment. In an embodiment, the position is a distance in thedirection substantially parallel to the direction of the optical axis ofthe projection system.

In an embodiment, the apparatus further comprises:

an actuator configured to vary the position between the first and secondparts; and

a controller configured to control the actuator to maintain apredetermined relative positioning between the first and second parts.

This may allow a relative position between the first and second parts ofthe projection system to be maintained, and reduce a likelihood ofmisalignment between the two parts reducing the image quality. Thecontroller may use feedforward or feedback control.

In an embodiment, the second part of the projection system is attachedto the liquid supply system. If the second part of the projection systemis attached to the liquid supply system, the construction of theapparatus may be simplified. The first part may then be fixed to areference frame, or another part of the lithographic apparatus asrequired. The second part is supported by the liquid supply system andtherefore any vibrations in the second part may not be substantiallytransmitted to the reference frame.

In an embodiment, the liquid supply system comprises a seal memberconfigured to seal liquid in at least part of the space between theprojection system and the substrate. It may therefore be possible tofill only a localized area with liquid.

In an embodiment, the seal member further comprises a contactless sealconfigured to seal liquid in the space.

In an embodiment, the second part is at least partly supported by aresilient member connected between the second part and a base frame.

In an embodiment, the base frame is decoupled from a frame to which thefirst part is attached.

According to a further aspect, there is provided a device manufacturingmethod comprising:

providing a liquid to a space between a substrate on a substrate tableof a lithographic apparatus and a first part of a projection system ofthe lithographic apparatus, a second part of the projection systemsubstantially decoupled from the first part; and

projecting a patterned beam of radiation, using the first and secondparts of the projection system, through the liquid onto a target portionof a substrate.

Transmission of vibrations to a first part of a projection system may besubstantially reduced or eliminated by mechanical isolation between thefirst part and a second part of the projection system.

In an embodiment, the method further comprises:

establishing a position between a first optical element in the firstpart of the projection system and a second optical element in the secondpart of the projection system; and

adjusting the position of the first optical element, the second opticalelement, or both such that the established position is maintained at apredetermined position.

By establishing a position and adjusting the position of opticalelements in the first and second parts of the projection system of alithographic apparatus, the position between the two optical elementsmay be maintained at a predetermined value. This method may allow theprojection system to be split at an arbitrary position, and maintain thecorrect separation between optical elements in the first and secondparts of the projection system. In an embodiment, the position is adistance in the direction substantially parallel to the direction of theoptical axis of the projection system.

In an embodiment, the method further comprises adjusting the relativepositioning of the first and second parts of the projection system tomaintain a predetermined relative positioning between them. Thus, therelative position may be maintained to reduce likelihood of amisalignment between the two parts that may reduce the image quality.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a cross-section of a proposed liquid supply system;

FIG. 3 depicts a plan view of the proposed liquid supply system depictedin FIG. 2;

FIG. 4 depicts a projection system and liquid supply system according toa first embodiment of the invention;

FIG. 5 depicts a projection system and liquid supply system according toa second embodiment of the invention; and

FIG. 6 depicts a projection system and liquid supply system according toa third embodiment of the invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g. DUV radiation), which in this particular case alsocomprises a radiation source LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to a first positioningdevice for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning device for accurately positioningthe substrate with respect to item PL;

a projection system (“lens”) PL (e.g. a refractive system) for imagingan irradiated portion of the mask MA onto a target portion C (e.g.comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning device (andan interferometric measuring device IF), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the beam PB. Similarly, the first positioning device can beused to accurately position the mask MA with respect to the path of thebeam PB, e.g. after mechanical retrieval of the mask MA from a masklibrary, or during a scan. In general, movement of the object tables MT,WT will be realized with the aid of a long-stroke module (coursepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. However, in the case of a stepper (asopposed to a step-and-scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at one time (i.e. a single “flash”)onto a target portion C. The substrate table WT is then shifted in the Xand/or Y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the projection system PL(typically, M=¼ or ⅕). In this manner, a relatively large target portionC can be exposed, without having to compromise on resolution.

FIG. 4 depicts a projection system and liquid supply system according toa first embodiment. The liquid supply system comprises a seal member 2which extends along at least a part of a boundary of the space 4 betweena final element of the projection system and a substrate table WT. Theseal member 2 is substantially stationary relative to the projectionsystem in the XY plane and a seal is formed between the seal member 2and the surface of the substrate W. In this embodiment, the seal is acontactless seal such as a gas seal and is formed by a gas bearing 18.The seal member 2 is supported above the surface of the substrate by thegas bearing 18.

A lower part 6 of the projection system is attached to the seal member 2by connecting members 8. An upper part 10 of the projection system isfixed by connecting members 12 to a reference frame RF. The referenceframe RF is connected by gas bearings 14 to a base frame BF of thelithographic apparatus. Further, gas bearings 15 support the base frameBF above the ground

Liquid is supplied via the seal member 2 to fill a space 4 between alower part 6 of a projection system and the substrate W. The gas bearing18 also functions as a seal to retain the liquid in the space 4 andprevent leakage of the liquid over the substrate W.

In use, the lower part 6 and upper part 10 of the projection systemfunction as a single lens system. Light from an illumination sourcepasses first through the upper part 10 and then through the lower part 6before finally passing through the liquid filled space 4 and hitting thesurface of the substrate W. In this embodiment, the projection system isa telecentric lens system. The separation between the optical elementsin the upper part 10 and the optical elements in the lower part 6 isdetermined by the location of the pupil (or aperture) in the projectionsystem. The separation point is located at the position of the aperture.This may be advantageous because at this point the light rays areparallel and the projection system is relatively insensitive tovariations in alignment in the Z direction (i.e. direction of theoptical axis, which is perpendicular to the substrate W). In anembodiment, the projection system is designed so that the aperture isalso between two lens elements having a large curvature radius. If theseparation point is between two lens elements having a large curvatureradius, for example plan plate or close to plan plate lenses, the systemmay be less sensitive to variations in the X and Y directions (i.e.parallel to the surface of the substrate W). In an embodiment, thesystem is separated between two lens elements having a curvature radiussuch that the sine of the incident angle, sin(θ), is less than 0.3.However, other curvature radius ranges are also possible, for examplesin(θ) can be less than 0.5 or less than 0.7.

During imaging, the substrate table WT moves the substrate W relative tothe liquid supply system and the projection system. This may produce acoupling force in the liquid filling the space 4 which may betransmitted to the lower part 6 of the projection system. However,because this lower part 6 is attached to the seal member 2 these forcesare transferred to the seal member 2. The seal member 2 is supported byresilient members 16. In this embodiment, the resilient members aremechanical springs. The springs provide some support to the seal memberand also act to damp any vibrations which are induced due to the effectof the forces.

The lower part 6 of the projection system is rigidly connected viaconnecting members 8 to the seal member 2. Therefore, if it is desiredto alter the position of the lower part 6 in the Z direction(perpendicular to the surface of the substrate W) the seal member 2 ismoved. This may be achieved by altering the operating pressure of thegas bearing 18 supporting the seal member 2. For example, to move theseal member 2 and lower part 6 upwards the pressure in the bearing 18 isincreased to create a net upwards force on the seal member which causesthe seal member to move upwards. When the desired position is reachedthe pressure is reduced to a steady state pressure (i.e. the forceexerted by the bearing is equal to the weight of the seal member) andthe seal member 2 and lower part 6 are then supported at the newposition.

The lower part 6 of the projection system is still subject to vibrationsarising due to coupling of forces via the liquid in the space 4.Therefore, it is advisable that the optical elements in the lower part 6are fixed. If the optical elements are free to move within the lowerpart 6, the effects of vibrations in the lower part 6 may induce aresonant vibration in one of the optical elements. This could have anadverse effect on the imaging quality, for example there may be a lossof contrast.

Conversely, the upper part 10 remains essentially vibration free. It issupported by the reference frame RF which is substantially isolated fromexternal vibrations to ensure the accuracy of measurement from sensorssupported on the frame. The upper part 10 may therefore include acombination of fixed or more loosely mounted optical elements dependingon the design requirements.

Although this embodiment has described separating the projection systemat the pupil of a telecentric lens, other separation points arepossible. For example, if the system is split between the two lenseshaving the largest curvature radius, the separation will be relativelyinsensitive to lateral alignment (i.e. parallel to the substrate). Aprojection system could be separated at this point and the apparatus canbe constructed without the need to align the two parts of the projectionsystem to very high tolerances. The projection system can also beseparated between any two lenses of large curvature radius, not just thetwo with the largest curvature radius.

It will be appreciated that the construction of this embodiment caneasily be adapted for various types of liquid supply apparatus, it isnot limited to the gas-sealed local area liquid supply system described.For example, the liquid supply system may contain the liquid in alocalized area by means other than a gas seal. Likewise, the liquidsupply system may immerse the whole substrate in a bath of liquid, andnot just immerse a localized area of the substrate.

Embodiment 2

A second embodiment of a projection system is illustrated in FIG. 5. Theconstruction of this embodiment is the same as for the first embodimentsave as described below.

In this embodiment, the separation in the projection system occursbetween an arbitrary pair of lens elements 22 and 24. To minimize theeffect of the vibrations induced by the coupling of forces throughliquid filling the space 4, only one lens element 22 is present in thelower part 6. Therefore, only one lens element is affected by theinduced vibrations. However, the projection system is more sensitive tomisalignment in a vertical, Z direction (i.e. perpendicular to thesubstrate) than at the separation position of the above described firstembodiment. It may therefore be advantageous to control the position ofthe lower part 6 with respect to the upper part 10 to maintain apredetermined distance between them. This distance control may be usedto give the correct focus to the projection system.

The predetermined distance may be maintained by measuring the distance dwith a sensor 23, for example an interferometer. The vertical positionof the lower part 6 relative to the seal member 2 is then be controlledusing one or more actuators 20 which connect the lower part 6 to theseal member 2. In an embodiment, the actuators 20 are Lorentz motors.However, they could also be Maxwell motors or other similar actuators. Asimple feedback controller using proportional, integral and derivativecontrol is used in this embodiment, but other types of controller arealso suitable. The controller compares the measured distance d to adesired distance and controls the actuators 20 to position the lowerpart 6 so that the desired distance is maintained. If the desireddistance d is not maintained the gain of immersing the substrate in aliquid may be lost, because the system will be out of focus in air andnot in the liquid.

In this embodiment, the distance between the two lens elements 24 and 22is calculated by measuring the distance d between the upper part 10 andthe lower part 6. The lens elements 22 and 24 are fixed and thereforetheir position with respect to the upper 10 and lower 6 parts of theprojection system is known. However, it is also possible to measure thedistance between the two lens elements 22 and 24 directly and use thisto control the actuator 20.

The control system may also control the position of the lower part 6 bya two-stage system if desired. In such a system the position of the sealmember 2 is adjusted by the gas bearing 18 as described above for thefirst embodiment. The actuator 20 is then used for fine relativemovements of the lower part 6 to the seal member 2.

While this embodiment has described controlling the vertical distancebetween the two lens elements 22 and 24, the control system can also beextended to control the relative position of the two parts: translationin the X and Y directions and/or rotations about the X axis and the Yaxis. This will further improve the accuracy and quality of theprojected image, particularly in the case where the lens elements 22 and24 have a small curvature radius and therefore are more sensitive tovariations in the X and Y directions. The relative position (translationand/or rotation) in the X-Y plane can be calculated by measuring theposition of the first and the second parts with interferometers.

Although this embodiment has split the projection system such that onlythe final lens element 22 is in the lower part 6, any other split ispossible. The lower part 6 can contain any number of optical elements,for example two, three, four, etc.

It will be appreciated that the control system of this embodiment can beapplied to the first embodiment to improve accuracy of alignment in thevertical, Z direction.

Although in this embodiment only the lower part 6 is moved, the controlsystem may also move the upper part 10 or both the upper 10 and lower 6parts to maintain the correct separation.

This embodiment enables a projection system to be split in an arbitraryplane. All of the optical elements which are located in the upper part10 are substantially isolated from vibrations induced by coupling forcesthrough the liquid filling the space 4.

Embodiment 3

A third embodiment is depicted in FIG. 6. The construction of thisembodiment is the same as for the first embodiment, save as describedbelow.

In this embodiment, one or more actuators 26 are attached to the upperpart 10 of the projection system. The actuators 26 are controlled by thecontrol system 28 to maintain a correct relative position of the upper10 and lower 6 parts of the projection system. This allows the accuracyto be further improved, because even in the case where the projectionsystem is split at a location which is relatively insensitive tomisalignment, there can still be some degradation of the image if thetwo parts are not aligned properly.

The control system 28 controls the actuators 26 to maintain apredetermined relative position of the two lens parts with six degreesof freedom: translation in the X, Y and Z axes and rotation about thesethree axes. It is also possible to use fewer degrees of freedom, forexample four, three, or two. The control system 28 has an input ofcalibration data and uses a feedforward control method. However, it alsopossible for the control system 28 to use feedback control with an inputof the measured relative position of the upper 10 and lower 6 parts in asimilar way to the above described second embodiment.

In an embodiment, to reduce vibrations in the upper part 10 of theprojection system during imaging, the control system 28 only operatesthe actuators 26 when the apparatus is not exposing a substrate.

Although the description of this embodiment has described moving theupper part to maintain a relative positioning of the upper and lowerparts, the same benefits can be achieved by moving the upper part tomaintain a relative positioning of the two lens elements adjacent to thesplit in the projection system.

It will be appreciated that this embodiment can also be combined withthe above described second embodiment.

All the embodiments can be applied to a system where only the substrateor where the entire substrate table is immersed in liquid, as well asthe localized area liquid supply systems described.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic apparatus comprising: a projection system including: afirst part having a first optical element, the first part arranged tocontact liquid in use, and a second part having a second opticalelement; a substrate table configured to hold a substrate and move thesubstrate relative to the first optical element of the first part; aliquid supply system configured to provide the liquid in a regionbetween the projection system and the substrate table; and a sensorconfigured to measure a positional relationship between the first partand the second part and/or between the first optical element of thefirst part and the second optical element of the second part.
 2. Thelithographic apparatus of claim 1, further comprising: a first memberconfigured to support the first part; and a second different memberconfigured to support the second part.
 3. The lithographic apparatus ofclaim 2, further comprising a frame supporting the first part and thesecond part.
 4. The lithographic apparatus of claim 1, wherein an imageof a pattern formed on the substrate is controlled by adjusting thepositioning between the first part and the second part.
 5. Thelithographic apparatus of claim 1, further comprising an actuatorconfigured to change the relative position between the first part andthe second part.
 6. The lithographic apparatus of claim 5, wherein theactuator is configured to change the relative position between the firstpart and the second part based on the measured positional relationship.7. The lithographic apparatus of claim 6, wherein the first partincludes the actuator.
 8. The lithographic apparatus of claim 6, whereinthe actuator is coupled between the second part and a frame supportingthe first part and the second part.
 9. The lithographic apparatus ofclaim 6, wherein the actuator is configured to adjust in at least twodegrees of freedom the relative position between the first part and thesecond part.
 10. A lithographic apparatus comprising: a projectionsystem including: a first part having a first optical element, the firstpart arranged to contact liquid in use, and a second part having asecond optical element; a substrate table configured to hold a substrateand move the substrate relative to the first optical element of thefirst part; a liquid supply system configured to provide the liquid in aregion between the projection system and the substrate table; and asensor configured to measure a positional relationship between the firstpart and the second part, wherein the second part is isolated from thefirst part such that substantially no vibrations are transmitted betweenthe first part and the second part.